It is a common practice to print letters, designs, figures and the like on the surface of a moulded resin article. The printing is performed by a variety of known techniques, from those using paints such as pad printing and impregnation printing, to a printing technique by means of laser beam irradiation (hereinafter, mentioned as laser marking). Although the methods using a paint are most widely employed for such printing, the processes have disadvantages: high processing cost, fears of environmental pollution by solvents, less possibility of recycling, etc. On the other hand, laser marking is a cheaper and more efficient method than the painting techniques owing to its simple marking process. Laser marking ensures excellent durability of the printed materials, thus being highly valuable in industrial applications. As a result, many techniques have been suggested for laser marking. For instance, Japanese Patent Application Laid-open No. 45926/1981 (JP-A-56-45926) teaches a technology for marking the surface of a moulded resin article made of a resin blended with a filler discolourisable by laser beam irradiation.
An example of the moulded articles to be printed with letters and figures by laser marking includes a keyboard of a personal computer. Main printing methods for the surface of a keyboard used to be pad printing onto an acrylonitrile-butadiene-styrene copolymer (an ABS resin) and impregnation printing onto polybutylene terephthalate (a PBT resin). However, in consideration of environmental problems inclusive of recycling as well as the factor of the cost, the marking method comprising irradiation of laser beams by using an Nd:YAG laser or other lasers on an ABS resin has been replacing the above processes.
On the one hand, a black or dark-coloured marking is generally effective on a moulded article coloured in light gray, cream or other colours having a high brightness or lightness. On the other hand, the black-marking does not stand out on a black or dark-coloured article. In the case of the latter, letters and figures should be printed in white or a whitish colour (hereinafter, referred to as a white marking). However, it is usually impossible to develop a white marking on an ABS resin by irradiation of laser beams.
Japanese Patent Publication No. 47314/1990 (JP-B-2-47314) discloses a marking technique for resins. The technique comprises irradiating laser beams on the surface of a thermoplastic resin such as an acrylic resin, whereby the irradiated part foams and lifts up to form a white-marked section. As methods of making a white marking on the surface of resins, there are other known methods which comprise incorporating, into a resin, titanium black [Japanese Patent Application Laid-open No. 81117/1988 (JP-A-63-81117), and Japanese Patent Application Laid-open No. 25806/1996 (JP-A-8-25806)], cordierite and/or mica [Japanese Patent Application Laid-open No. 267191/1992 (JP-A-4-267191)], aluminium hydroxide [Japanese Patent Application Laid-open No. 25317/1993 (JP-A-5-25317)], and other inorganic compounds. These methods are still impractical in terms of the whiteness and quality of the marked letters and figures, and also in terms of the impact resistance, which has decreased as a result of the incorporation of the inorganic compounds.